This invention relates to a fluid operated actuator device utilising a primary rotational movement.
In conventional fluid operated actuators, for example 90 degree valve actuators, the primary movement is the linear motion of a piston and cylinder, which has to be transferred into a secondary rotational movement to actuate the valve. Such arrangements require a surplus of moving parts, so they can be complex in structure and hence difficult to service and use.
In known 90 degree valve actuators the linear movement of a piston inside a cylinder is transferred into a rotational movement by a linkage rod attached at its inner end to the piston, and attached via an actuator linkage at its outer end to a turning arm mounted on a rotatable shaft. The rotating movement of the actuator linkage provides uneven torque to the shaft, which operates the valve. When the piston is at the bottom of the cylinder and the valve is in the fully shut or fully open position the actuator linkage is wider than 90 degrees. As the piston moves up the cylinder the linkage closes, and is at 90 degrees when the piston is approximately half way up the cylinder. The linkage continues to close until the piston is at the top of the cylinder and the linkage reaches its narrowest angle, which is less than 90 degrees. The torque applied to the linkage is at its greatest when it is at 90 degrees and the full force of the piston is applied straight down the linkage rod to turn the valve shaft. The torque decreases when the linkage is wider or narrower than 90 degrees, at the beginning and at the end of the stroke.
This variation in torque causes a particular problem for valve actuators, because the valves are known to stick shut during operation. So-called “break out torque” is required to re-open the valve and resume operation. Break out torque is greater than the torque required to actuate the valve normally, and is the highest torque required of the valve actuator at any time. Therefore, a valve actuator powered by the linear movement of a fluid operated (hydraulic or pneumatic) piston and cylinder as described above must be able to provide the required break out torque at the beginning or the end of the stroke, when the torque provided is at its lowest. This means that a piston and cylinder arrangement is required which provides an excess of power in the middle of its stroke.
Valve actuators are used in industries which require a high level of sanitation, for example at chemical or food processing plants. As a result pneumatic cylinders are preferred to hydraulic arrangements because of the reduced risk of contamination. Further, it is desired to have a valve actuator constructed wholly from stainless steel because this is more readily sanitised than other materials, for example aluminium.
Many known types of valve actuators use spring loaded piston and cylinder arrangements which can be hazardous to assemble for use and disassemble for cleaning and maintenance.